Trigger system

In designing the NA61/SHINE trigger system, particular attention was paid to developing a ﬂexible and robust system capable of handling and selecting different reactions using a variety of beams (pions, kaons, protons, ions) and targets as required by the NA61/SHINE physics programme. The trigger is formed using several of the beam counters, the Cherenkov detectors for beam particle (hadrons or ions) identiﬁcation, and the PSD calorimeter. The core of the trigger logic is an FPGA running at 120 MHz embedded in a CAMAC Universal Logic Module. CAMAC is used for backward compatibility with the legacy NA49 electronics. The trigger logic is divided into three main blocks:

beam logic

beam particle identiﬁcation

interaction logic.

Up to four different triggers can be run simultaneously with a selectable 12 bit pre-scaler or each trigger. Different trigger conﬁgurations are recorded in a pattern unit on an event-by-event basis for off-line selection. Analog signals from the beam counters are ﬁrst discriminated with constant fraction and leading edge discriminators before entering a second discriminator, whose role is to shape the logic signals (12ns width) and convert them to ECL levels, as required by the FPGA trigger logic. These logic signals are also recorded in pattern units on an event-by-event basis for verifying the trigger logic in the analysis of trigger data. The combined use of two discriminators with different output widths prevents also the pile-up in the trigger logic (the length of the output of the ﬁrst discriminator is around 100ns, while the length of the second discriminator is 12ns). Correspondingly, the dead time of the trigger system is around 100ns, small compared to the dead time gated trigger rate of 100Hz.